The invention relates to a cellular telecommunications system in accordance with the preamble of Patent claim 1.
Cellular telecommunications systems which are already known are generally more cost-intensive the bigger their transmission/reception range, i.e. the more cells there are in the cellular telecommunications system. This is primarily due to the fact that each individual cell is assigned its own base station. Therefore, the greater the area which is to be covered by a cellular telecommunications system, the more base stations are necessary. Consequently, in contemporary cellular telecommunications systems the base stations constitute one of the main cost factors.
The object of the invention is to provide a more cost-effective cellular telecommunications system with which a greatest possible transmission/reception range can be made available.
The means of achieving the object which has been set can be found in the characterizing part of Patent claim 1.
A cellular telecommunications system in accordance with the invention is characterized by the fact that all n cells are assigned only one common base station with n antennas which can be switched over, the number n of cells being greater than or equal to two.
According to a preferred exemplary embodiment of the invention, the telecommunications system has precisely two cells, the assignment of the cells to the common base station being carried out by means of a switch which is contained in the base station and via which the respective antennas are connected to a RF (Radio Frequency) unit contained in the base station. In this way, it is possible to make a saving of one base station in comparison with conventional telecommunications systems, which has positive effects on the costs.
By making an appropriate selection of the number of antennas and of the switch, it is possible, as an alternative, also to assign more than two cells to one base station in each case.
According to one development of the invention, the switch is controlled by a microcontrol unit (MCU) contained in the base station. The switch can be controlled here in accordance with an algorithm which is implemented, for example, as software in the MCU. In this way, the control of the switch can be adapted relatively easily and quickly to different demands.
According to another development of the invention, information which is supplied by the MCU and which specifies in which time slot and at what frequency the RF unit has to transmit data is buffered in a BMC (Burst Mode Controller) before it is passed on to the RF unit. It is necessary to buffer the data because, depending on the position of the switch, the data can be passed on to the RF unit only in corresponding time slots.
So that the MCU can carry out the abovementioned functions, according to one development of the invention said unit is coupled to memory modules which may be designed, for example, as fixed and/or variable ROMs (Read Only Memory) and/or RAMs (Read Access Memory), the components which are contained in the base station being synchronized by means of a common clock generator. This ensures that the correct data are transmitted via the correct antenna in the corresponding time slots.
According to yet another refinement of the invention, a first antenna is arranged in a building and a second antenna is arranged outside the building. In this way, the transmission/reception range of the telecommunications system can easily be extended. Consequently, it is possible, for example, also to set up a connection between two mobile stations, one of which is located in the interior of the building and the other in the garden, for example.
Telecommunications systems of the abovementioned type can, for example, be operated according to the DECT Standard. These may be cellular or so-called cordless systems.
According to the DECT Standard, it is possible to transmit both voice and data signals. Consequently, it is possible to set up cellular data networks which are based on the DECT Standard and which permit the subscriber to move freely within various cells without, for example, the call which is currently being made being interrupted. Voice and data signals are transmitted here by means of so-called bearers (see DECT Standard for more details).
In order to prevent a call being interrupted, a call is, when necessary, transferred from one cell into another, which is customarily also referred to as handover, (changing over to another channel), specifically also as intercell handover.
A handover can also be carried out within a cell (intracell handover) if the connection quality is, for example, degraded as a result of common channel interference. Here, a change-over of the radio channel takes place within a cell with the same base station being retained. In this context, the change-over of the channel can consist in changing over between frequencies or between time slots.
In the abovementioned types of handover, a channel change is seamless, that is to say without the connection being interrupted. This is achieved by virtue of the fact that the old bearer is not released until the new one has already been set up. Consequently, two bearers must be operated in parallel for a short time, which leads to an increased loading of the system. However, in order to protect the resources in a telecommunications system, the number of handovers must, on the one hand, be kept as low as possible, while, on the other hand, any desired reduction in the number of handovers is not possible since otherwise the desired connection quality cannot be maintained.
In order, to be able to utilize fully the power of, for example, a two-cell telecommunications system according to the DECT Standard, so-called dummy bearers (DB) must be transmitted in each cell via respective antennas.
So the mobile stations know that a call has arrived for them, they must activate their receiver in a specific time slot and at a specific frequency in order to determine whether there is information for them. Customarily, they receive this information via the broadcast channel which is usually transmitted via the dummy bearer (DB).
In order to avoid the dummy bearer being subject to interference, it must be transmitted on various channels, i.e. at a different frequency and/or in another time slot.
If, for reasons of cost, a so-called low-cost synthesizer is used in a base station, it is not possible with such a synthesizer to change the frequency in two successive time slots since the synthesizer is too slow. This means that, instead of the 24 time slots defined in the DECT standard, it is possible to use only every second time slot. Consequently, when a xe2x80x9clow-costxe2x80x9d synthesizer is used, only six time slots are used in the downlink (base stationxe2x80x94mobile station) and six in the uplink (mobile stationxe2x80x94base station), respectively.
If it is then desired to reduce the number of base stations in the abovementioned example from two to one and to transmit the respective bearers into both cells via the remaining base station in which a slow synthesizer is being used, in order to set up the bearers (traffic bearers and dummy bearers) there are therefore only six time slots left for connections from the base station to the mobile station and from the mobile station to the base station. In the case of a standard configuration of a telecommunications system it should be possible to make telephone calls to the outside (line call) from a cell using at least one mobile station, while being able at the same time to set up a connection to a mobile station of another cell (inter-call) using a different mobile station in the cell. Furthermore, a handover must also be possible in each cell, it being possible for further mobile stations to be in stand-by mode. As a result, the following number of bearers results for a telecommunications system according to the DECT Standard with two cells and xe2x80x9clow-costxe2x80x9d synthesizer:
This means that it is not possible to implement a two-cell telecommunications system according to the DECT Standard using only one base station if a xe2x80x9clow-costxe2x80x9d synthesizer is used in the base station.
Of course, it will be possible to conceive of using in this case a faster synthesizer in which twelve channels can be used in the uplink and in the downlink, respectively. However, such a synthesizer is extremely expensive.
If, on the other hand, a telecommunications system with more than two cells is considered, the abovementioned problems relating to channel capacity arise even when a high-speed synthesizer is used.
Therefore, in a development of the invention, a bearer transmitted on a first channel contains information relating to the position of a second channel so that the mobile stations of the respective cell can set themselves to the second channel using this information. In this context, position means information relating to the frequency and time slot. In this way, a xe2x80x9cseamless handoverxe2x80x9d, for example, can be implemented in which there is no interruption in the call during a handover (intercell or intracell).
According to yet another refinement, the position information on the first channel is transmitted parallel to a traffic bearer (TB) which has been set up on the second channel. The mobile stations of the cellular telecommunications system are thus selectively informed as to which time slot and which frequency they should set themselves to in order to receive broadcast information. The broadcast information contains information relating to when the mobile stations must activate their respective receiver (at which frequency and in which time slot). As a result, the mobile stations do not have to search through all the channels individually for broadcast information, which reduces the power consumed by the mobile stations considerably.
According to a further development of the invention, the position information is transmitted via a dummy bearer which is switched off after a predetermined time has elapsed. As a result, the channel seized by the dummy bearer is released and can be used for other purposes, for example for setting up a further traffic bearer.
According to one preferred embodiment, the dummy bearer is switched off after 640 milliseconds. During this time, all the mobile stations set to the dummy bearer are informed about a change of channel. If no change of the broadcast channel is expected, the mobile stations activate their receiver every 640 milliseconds, which corresponds to the length of four multiframes, which are each 160 milliseconds long. Since the base station broadcasts the broadcast channel in each multiframe in each case, it is ensured that all the mobile stations are informed about the new channel before the dummy bearer is switched off.
Alternatively, the dummy bearer (DB) may also be switched off only after any other desired appropriate period of time. Thus, the dummy bearer may be maintained, for example, until the physical channel seized by it is required for other purposes, for example to set up a further traffic bearer. In this way, all the available channels of a DECT telephone system are used to an optimum degree.
According to a further embodiment of the telecommunications system according to the invention, only the dummy bearer is transmitted on the first channel. Consequently, the broadcast information transmitted to the mobile stations is transmitted via a permanent dummy bearer as long as no traffic bearer (TB) has been set up. After at least a first traffic bearer (TB) has been set up, all the mobile stations synchronized on the dummy bearer (DB) receive broadcast information relating to the change of channel via said dummy bearer (DB). In this way, the mobile stations can be selectively informed as to which time slot and which frequency they must set themselves in order, for example, to receive further broadcast information.
This broadcast information may be contained, for example, in the newly set-up traffic bearer to which all the mobile stations now set themselves, even if they do not require a traffic bearer themselves at the moment. As a result, the channel seized by the dummy bearer is released immediately after a first connection of a mobile station is set up.
According to a different embodiment of the telecommunications system according to the invention, a traffic bearer (TB) is transmitted on the first channel and, after the traffic bearer has been set up on the second channel, said first traffic bearer (TB) is transformed into the dummy bearer. The conversion of a traffic bearer into a dummy bearer can be achieved, for example, by stopping only the voice transmission (decoupling of the U plane). Stopping the voice transmission is recommended since, otherwise, the other mobile stations set to the traffic bearer may possibly be disrupted.
According to yet another embodiment of the telecommunications system according to the invention, when the last traffic bearer (TB) is switched off it is converted into a permanent dummy bearer (DB) so that only voice transmission has to be stopped on this channel. In this case, too, it is not necessary for the mobile stations to scan the channels for the broadcast channel.
Furthermore, channels of cells may be assigned in each case to other cells. As a result, it is possible to avoid temporary channel bottlenecks of a cell as a result of unused channels of a different cell, unused channels being concealed in the other cell.